fix: Telegram listener — async command execution + singleton guard

Root cause: run_and_reply() blocked the entire listener synchronously.
FFmpeg hung because child processes inherited stdin (waiting for 'q').
Long-running commands froze the listener for up to 120s.

Fix:
- Commands run in background with stdin=/dev/null, output to temp file
- reap_commands() collects output non-blocking after each getUpdates cycle
- SIGCHLD handler pre-caches exit codes via wait -n
- TERM/INT trap kills background processes and cleans temp files
- Singleton guard (flock) prevents duplicate listeners racing getUpdates

Tests:
- t-telegram-listener-exec.sh: 12 hermetic checks (echo, pipes, stderr,
  compound commands, long-running, quiet mode)
- t-telegram-listener-singleton.sh: 8 checks (lock acquire/release/status)

Architect verdict: accepted as-is, no re-architecture needed.

DOC/AGENT_Context_Project.md

@@ -640,7 +640,7 @@ Use conventional prefixes: `feat:`, `fix:`, `docs:`, `refactor:`, `chore:`
 | `bin/pos-communication-matrix-listener` | 582 | Matrix listener: map /command → bash, run them on room messages |
 | `bin/pos-communication-matrix-sender` | 215 | Send messages to a Matrix room via the client-server API (send, test, login) |
 | `bin/pos-communication-scrcpy` | 245 | Mirror/control an Android device via scrcpy+adb (mirror, devices, record, tcpip, connect, push, pull, screenshot, info) |
-| `bin/pos-communication-telegram-listener` | 815 | Telegram bot listener: map /command → bash and <prefix> → app, run them on chat messages |
+| `bin/pos-communication-telegram-listener` | 889 | Telegram bot listener: map /command → bash and <prefix> → app, run them on chat messages |
 | `bin/pos-communication-telegram-sender` | 212 | Send Telegram messages/files/links/stickers via Bot API (send, test) |
 | `bin/pos-docker-compose` | 487 | Docker Compose service manager (ls/up/down/restart/logs/update/config) |
 | `bin/pos-docker-health` | 107 | One-glance container health dashboard (exits 1 if unhealthy) |